1. Field of the Invention
The present invention relates generally to a portable radio communication system. More specifically, the invention relates to a diversity type portable radio communication device which has a dual receiver system.
2. Description of the Related Art
In the conventional portable radio communication system, diversity reception by a dual receiver system is performed for compensating phasing in mobile communication. In such dual receiver system, electric field level is detected by respective of two receivers in the dual receiver system and converted into direct currents so as to compare the received electric field levels for selecting one of the receivers having higher received electric field level for receiving data.
Accordingly, in this construction, the portable radio communication device is operable by maintaining power supply for both of the receivers even under a condition where electric field intensity is stable.
FIG. 4 shows one typical example of the conventional portable radio communication device. As shown, the dual receiver system comprises two receivers 3 and 4, antennas 1 and 2 respectively connected to a corresponding one of the receivers. A field level detector circuit 5, a control portion 6 and power source portion 8 supply power to the receivers 3 and 4.
The receivers 3 and 4 convert electric field intensity of a signal received through the antennas 1 and 2 into direct currents having direct current values 11 and 12. The direct current values 11 and 12 are output to the electric field level detector circuit 5. The electric field level detector circuit 5 detects the electric field levels of respective of the receivers 3 and 4 on the basis of the direct current values 11 and 12 to output a detection signal 19 to the control portion 6.
The control portion 6 performs comparison of the electric field levels corresponding to the direct current values 11 and 12 by the detection signal 19. On the basis of the result of comparison, the control portion 6 selects one of the receivers 3 and 4 having higher electric field level. Then, the control portion 6 switches reception channel to the selected one of the receivers 3 and 4 to receive data therethrough. It should be noted that the power source 8 constantly supplies electric power 15 and 16 to both of the receivers 3 and 4 for enabling them.
Therefore, even when the control portion 6 compares the electric field levels corresponding to the direct current values 11 and 12 with a preliminarily set electric field level and makes judgement that the received electric field is a stable and strong electric field, the electric power 15 and 16 is supplied to the receivers 3 and 4 from the power source 8.
As set forth above, in the conventional portable radio communication device, the electric field levels detected by respective of two receivers in the dual receiver system are converted into the direct current values, the direct current values are compared with each other so that one having higher electric field level is selected for obtaining received data.
Therefore, even in the stable state where the electric field is strong and thus no drop of the electric field occurs, it is inherent to keep both receivers ON. Therefore, unnecessary power consumption is inevitable.
Another type of diversity type portable radio communication device has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. Showa 63-33021, for "Portable Radio Communication Device". In the disclosed portable radio communication device, in order to provide a battery saving function is provided for the device, in the portable radio communication device which can have two stand-by modes, i.e. an in-area stand-by mode which is normal stand-by mode within a service area and an out-area stand-by mode which is a stand-by mode out of the service area, a detection circuit is provided for determining whether the portable radio communication device is present within or out of the service area, and a battery saving circuit for periodically supplying power to the respective receivers at given intervals when judgement is made in the out-area stand-by mode and constantly supplying power to both receivers in the in-area stand-by mode.
As can be appreciated, the shown portable radio communication device determines whether the device itself is present within the service area or not so that the power is supplied to the receivers periodically and alternately in the out-area stand-by mode. As set forth, since the diversity type portable radio communication device requires that both receivers are kept ON, the system disclosed in the above-mentioned publication is not applicable for the diversity type device.